The proposed Xiaonanhai Dam would stand athwart the mighty Yangtze River some 30 kilometers upstream of the industrial metropolis of Chongqing. The dam represents the single largest project in that municipality's 11th five-year plan, costing roughly $3.5 billion. The growing city hopes to harvest 1.7 gigawatts of electricity from the river current of the Chang Jiang, as the Chinese call the third longest river on Earth.

The dam is just one of 19 proposed dams on the upper reaches of the Yangtze, upstream from the massive Three Gorges project near Yichang—and one of nearly 200 proposed dams on the Yangtze and its tributaries. China is building hydropower at a record pace that has resulted in more dams—26,000—than in any other country. In the last decade more than 60 percent of all hydropower projects worldwide were in China and Three Gorges alone will avoid the emission of an estimated 95 million metric tons of carbon dioxide a year—more than the combined output of Norway and Sweden.

"Within 30 to 50 years, hydropower will be the main energy we should rely on," Lai Hun Suen, a professor of sustainable development at Chongqing University and municipal government official, explained to me during a visit to the city in 2008. "It is a choice we made when we had no other choice."

That choice has consequences: At least 1.2 million people moved to Chongqing, displaced by the rising waters behind Three Gorges. Decomposing submerged vegetation burps methane—a greenhouse gas which traps 25 times as much heat as CO2 over a century. And a planned dam, Xiaonanhai, that would be built in the middle of the last remaining untouched habitat of the Yangtze sturgeon, giant salamander and 66 other fish species of concern, says Yan Xie, China Program director for the Wildlife Conservation Society. In fact, the proposed dam borders the "core protected area" of the Upper Yangtze Native and Rare Fish Reserve—an area set aside to mitigate some of the impacts of the construction of Three Gorges Dam.

"There are 338 kinds of freshwater fish in the Yangtze River and 162 of them are endemic to the river—that is, found nowhere else," says ecologist David Dudgeon of the University of Hong Kong. "It seems obvious that many or all of them could be impacted by a dam in the middle of the reserve."

A drop in the bucket
Xiaonanhai would generate 1,760 megawatts of electricity—three quarters the yield of the U.S.'s Hoover Dam, but just eight percent of Three Gorges potential 22 gigawatts, and three percent of the 60 gigawatts from the proposed dams planned along the upper Yangtze, also known as the Jinsha River.

That's why The Nature Conservancy is working on a plan that would invest Chongqing's dam-building money into making the other dams upstream slightly bigger. "They could own as much as three times the kilowatts as they could ever get out of the little dam and keep all the fish," says lawyer David Harrison, a senior advisor to the Conservancy's Global Freshwater Program. "Chongqing could get the economic development they want, and a fish reserve. They may not appreciate it now but they will, as will their children."

Already, the Chinese paddlefish, a six-meter-long endemic species, has not been spotted in more than three years in the region and may now be functionally extinct, much like the baiji—a freshwater dolphin—downriver. "Four famous domestic fishes"—the grass, silver, black and big carps—provide protein and a livelihood to more than 10 million Chinese, yet they are dwindling in the stretch of river beneath the Three Gorges Dam, victims of the dam's development and overfishing, says Daqing Chen of the Yangtze River Fisheries Research Institute.

The solution, according to the Conservancy, is to plan hydropower development on a watershed level rather than allowing the proliferation of projects from various levels of government: county, township, city, provincial and even national. "There's wildcat dam development going on. At some level, it's out of control," says Brian Richter, director of the environmental group's Global Freshwater Program, adding that the proliferation of dams of all sizes are "just getting built in an uncoordinated fashion."

As a result, a coalition of Chinese scientists and environmentalists signed a petition this May calling for a halt to "overdevelopment" of hydropower in the Yangtze River Basin. The central government's Ministry of Environmental Protection responded in June, suspending the Xiaonanhai project pending further review—along with two other nearby dams in the Jinsha River already under construction.

The problem is not dams per se. It is the lack of planning. After all, building hydropower will also enable the proliferation of intermittent renewable energy sources, such as wind and solar. "Xiluodu [Dam] could be used to stabilize against wind and solar to a very large degree because of its size and because it's got another reservoir downstream that can dampen out any fluctuations," Harrison notes. "How many kilowatts of other renewables can you enable if you develop smartly? Building small dams in small places doesn't help you with that."

Flood control
Of course, there is also the specter of floods. Inundations in 1931 and 1954 killed hundreds of thousands along the lower Yangtze and a similar flood in 1998 displaced at least 1.8 million people. Therefore, one of the primary reasons to build Three Gorges Dam, in addition to hydropower and improved shipping channels, was to restrain the periodic, devastating flooding of the Yangtze.

But a river basin regime that also restores some of the natural floodplains—without displacing the rich rice paddies and vegetable fields that provide 60 percent of the country's food supply—could also generate more power. After all, preparing for floods means lowering water levels in the reservoir to allow for extra water, resulting in reduced hydropower output. Allowing the Three Gorges reservoir to remain at a higher water level, for example, by opening some flood storage downstream could "generate $1 billion more in hydropower revenue," Richter argues.

The key would be to inundate several hundred thousand hectares of the agricultural lands periodically during times of flood to provide as much as 15 billion cubic meters of floodwater storage—three-quarters of the 20 billion cubic meters of flood control offered by the Three Gorges reservoir itself.

But it would also involve temporarily relocating the inhabitants of these lands through early warning and compensating them for lost harvest—something China already does, such as during the 1998 flood. And some of the $1 billion in extra hydropower revenues could go into a proposed compensation fund for such displacement and lost crops, Richter argues, a plan for which Goldman Sachs is preparing a feasibility study. "Using floodplains to store floods is more reliable, safer and more economical," Richter says.

Such a plan would also more closely mimic the natural flow of the river, which is naturally high during spring and summer and low in winter, the opposite of the flow pattern imposed by dams such as Three Gorges that attempt to reserve room for floodwaters. "Fish reproduction in the Yangtze and other rivers depends on the maintenance of natural flow patterns, because it is these that set the cues for breeding and stimulate breeding migrations," Hong Kong's Dudgeon notes.

Such river basin regimes could prove vital in other areas of the world undergoing a hydropower boom, as well, Richter says. In Central America there are more than 300 dams of varying sizes planned or under construction. "What we're seeing is a very rapid proliferation of hydropower development in these countries," he says. "It's a race to build dams and get them hooked up as quickly as possible, because the first in line are the first to be rewarded in terms of revenue."

And it might help ameliorate or avoid some of the other impacts from dam construction now coming to the surface. "More than 100 miles of [Three Gorges] reservoir banks are at risk of collapsing. More than 50 percent of the reservoir area is affected by erosion and an additional 500,000 people will need to be relocated," says Peter Bosshard, policy director at International Rivers, a dam watchdog group. Further downstream there is also erosion in the Yangtze's delta as well as seawater intrusion. "Shanghai is subsiding. All these factors need to be taken into account."

Despite these concerns, dam construction continues. Coffer dams—a temporary barrier that allows work on a permanent dam to proceed—are already in place for many projects, such as the Ahai Dam on the Jinsha River, closing off yet more sections of the waterway.

"It's not just dams versus no dams," Harrison adds. "It's about elegant dams—and if we can't do that, we shouldn't build them. The proliferation of small- and middle-size ones is chewing up the whole place."

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.